1. Field of the Invention
The present invention relates to data storage media, and more particularly relates to a method and apparatus for measuring the spacing between adjacent tracks in a data storage medium. The invention is especially adaptable to track pitch measurement in connection with optical discs.
2. Brief Description of the Prior Art
The optical disc is a high density information storage medium which is used to record television program material for later playback as well as for other uses such as computer data storage.
Optical discs are typically made of a durable, clear plastic material formed in a disc approximately the size of an LP phonograph record. Information is stored inside the disc in an embedded surface having a uniform optical property such as reflectance. Tiny indicia, or areas in the surface having the uniform optical property altered, for example having decreased reflectance, are sequentially arranged on the surface in the form of tracks. These tracks are arranged as concentric circles or as a single continuous spiral. Information is represented in the variation in the spatial frequency of the indicia in the track, as well as in the relative size of the indicia as compared with the "lands" of unaltered surface between them. For example, one common form of optical disc, used in the recording of video information, has a reflective surface embedded in the interior of the disc, in which the indicia are tiny pits.
The disc is read by a player apparatus which images a tiny "reading" spot of light onto a track and moves the spot of light relative to the track. The reflected signals modulated by the pits and lands are gathered, and the stored information is recovered from the reflected light signal. The spot of light is kept centered on the track by means of a servo system which utilizes two auxiliary spots of light stradling opposite sides of the track, in a manner known in the art. A detailed description of such an optical disc player can be found in co-pending U.S. patent application Ser. No. 131,513, commonly assinged to the assignee of the present invention.
Optical discs provide high density information storage by virtue of the extremely small size of the information-containing indicia which make up the tracks of the disc, and the small track-to-track spacing, or "pitch". Such indicia typically vary in length in a range from approximately 0.75 micron to 1.25 micron. Track pitch is typically of the order of 1.5 to 2.0 microns. The reading spot of light for optical discs is correspondingly small. In order to be able to retrieve the information stored on the disc it is important that the reading spot of light be maintained centered on the track and that the tracks be sufficiently spaced from one another that the amount of light which is reflected off of adjacent tracks is kept to a reasonable minimum.
In this connection, it is known that it is impossible to create such a tiny spot of light which has perfectly sharp borders. Therefore, even if a circular spot of light having a diameter less than a track width is perfectly centered on a track, some light from the spot will impinge on adjacent tracks and thereby create a signal component from the adjacent tracks. This component is called "crosstalk", and if it is large enough it can cause objectionable interference with the receovered information signal. It is, therefore, important that the track pitch be chosen so as to provide an acceptably low level of crosstalk and that that track pitch be maintained at a farily constant value. In addition, it is important that abrupt deviations in track pitch be avoided, because such deviation can cause the tracking servo to exceed its limits resulting in picture dropout or skipping from track to track.
The aforementioned track pitch deviations in optical discs are produced by more than one source in the disc manufacture process. In addition, the characteristics of the deviations vary with the particular source. For example, the formation of the tracks of indicia in the manufacturing process is typically done using a "writing" spot of laser light which exposes a layer of photo-resist, melts holes in a layer of metal, or the like, in the pattern the tracks ultimately take. Oscillations in the objective lens tracking servo system for the "writing" laser can give rise to track pitch variations having a relatively high spatial frequency. On the other hand, eccentricities in the lead screw, which is used to drive the objective lens assembly radially to cause the tracks to spiral, can give rise to track pitch variations having a significantly lower spatial frequency than those from the objective lens servo system.
In order to monitor the quality of the optical disc manufacturing process, it is, therefore, desirable to have the capability to measure track pitch deviations with a sufficient degree of accuracy so as to be able to not only detect the presence of track pitch variations, but also to detect the characteristics of those variations, for example, spatial frequency. Knowledge of the characteristics of track pitch variations can aid in pinpointing those defective or marginal components which are their source. Such knowledge can also be used to ascertain whether the track pitch variations are of an unacceptable magnitude.
Until now, however, the method employed in commercial optical disc manufacture to detect track pitch variations directly has been to put the disc under a microscope and have an individual look at various areas on the disc. This has not proved to be a very satisfactory technique because the disc is very large compared with the field of view of the micrscope and, consequently, inspecting the entire disc is extremely time-consuming. In addition, track pitch variations of significance may occur in spatial patterns which are relatively large as compared with the microscope field of view. While it may be possible to detect the existence of an individual track pitch variation of this kind, patterns of occurrence of such deviations may easily elude the observer. In addition, because the visual field of the microscope covers such a tiny portion of a group of tracks, it is difficult to compare observed track deviations between different observations through the microscope, even those covering relatively closely spaced fields, so as to correlate observed deviations and thereby obtain information concerning deviations of a particular track.
It will, therefore, be appreciated that there is a need for an apparatus capable of providing accurate measurements of variations in the spacing of adjacent tracks in an information storage medium such as an optical disc. Further, there is a need for an apparatus capable of providing accurate measurement of track pitch in an optical disc over the entire information area of the disc in a relatively short amount of time.
The present invention satisfies these needs.